1. Field of the Invention
The present invention relates to a nonvolatile memory device and, more specifically, to a phase-change memory device, which is phase-changed according to an applied voltage or applied current, and a method of fabricating the same.
2. Discussion of Related Art
In general, some of materials formed of elements of Groups 14 to 16 have phase-change characteristics sensitive to light or a current pulse. Chalcogenides, which are formed of chalcogens such as sulfur (S), selenium (Se), and tellurium (Te), are typical examples of phase-change materials, and it is known that germanium antimony (GeSb) and zinc antimony (ZnSb) exhibit phase-change behaviors.
A phase-change material has a crystalline phase and an amorphous phase according to a crystalline structure. Since the optical reflectance and electrical resistance of the phase-change material depend on the phase thereof, the phase-change material may be used to fabricate an optical storage medium or a nonvolatile memory device.
Optical disks, such as digital versatile disk rewritables (DVD-RWs) and digital versatile disk-random access memories (DVD-RAMs), which are based on the optical properties of phase-change materials, have already been put to practical use. In addition, a vast amount of research has been conducted on putting nonvolatile memory devices using electrical properties of phase-change materials to practical use.
A nonvolatile memory device makes use of a difference in resistivity of a phase-change material between an amorphous phase and a crystalline phase. The phase change of the phase-change material is induced by the application of a current pulse. The amplitude and duration of a current pulse applied to the nonvolatile memory during a SET transition in which an amorphous phase is switched to a crystalline phase are different from those of a current pulse applied to the nonvolatile memory device during a RESET transition in which the crystalline phase is switched to the amorphous phase.
Since it is only necessary to heat the phase-change material to a crystallization temperature or higher to enable the SET transition, the amplitude of a current pulse need not be so large. However, since some time is needed for crystallization, the application of the current pulse must be continued for a critical amount of time or longer.
In comparison, it is necessary to heat the phase-change material to a melting point or higher to enable the RESET transition. Accordingly, the amplitude of a current pulse must be even larger than for the SET transition, and a time taken to apply the current pulse must be reduced to be shorter than for the SET transition in order to inhibit the crystallization of the amorphous phase-change material.
A very large amplitude of a current pulse required to induce a RESET transition becomes a serious obstacle to the use of a phase-change memory device in practice. When a typical phase-change material layer formed of GeSbTe (GST) is used and a contact area between the GST phase-change material layer and a lower electrode is 0.5×0.5 μm2, a current pulse of about several mA or more must be applied to enable a RESET transition. In order to reduce a RESET current, a method of changing a device structure or a method of adopting a new phase-change material or a new lower electrode material is being considered.
FIG. 1 is a cross-sectional view of a conventional memory device having a phase-change material. Referring to FIG. 1, the conventional phase-change memory device includes a lower electrode 106, a heating layer 108, a phase-change material 112, and an upper electrode 116.
When power is applied between the lower electrode 106 and the upper electrode 116 of the phase-change memory device, a portion of the phase-change material 112 is phase-changed. In this case, the phase-changed portion is referred to as a programmable volume 112a. As the programmable volume 112a of the phase-change memory device decreases, a current required for a SET or RESET transition decreases.
In order to reduce the programmable volume, a method of downsizing a contact hole which the phase-change material 112 contacts the heating layer 108 may be used. However, the method of downsizing the contact hole involves high-cost semiconductor process technologies.
Specifically, advanced photolithography and etching techniques are required to form sub-micron contact holes. Also, a deposition technique with good step coverage characteristics is required to fill the sub-micron contact holes.
Accordingly, it is necessary to develop a method of fabricating a phase-change memory device that does not require advanced process technologies or incur high fabrication costs.